Electrical control systems



Dec 4, 1956 R. w. MOORE ETAL 2,773,228

ELECTRICAL CONTROL SYSTEMS INVENTORS $1- Raymond W.Moore,Robert E.Hu||

IW/ ond Loren F.Stringer.

ATTORNEY WITNESSES:

Dec. 4, 1956 R. w. MOORE ET AL ELECTRICAL CONTROL SYSTEMS 2 Sheets-Sheet 2 Filed Oct. 30. 1953 INVENTORS 1Robert EHull n F. Stringer.

WITNESSES:

Reymond WAMoore ond Lore ATTORNEY United States Patent O ELECTRICAL CNTROL SYSTEMS Raymond W. Moore, Snyder, and Robert E. Hull, Bulalo, N. Y., and Loren F. Stringer, Pittsburgh, Pa., assiguors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsyivauia Application Detober 30, 1953, erial No. 389,496

26 Claims. (Cl. 318-87) This invention relates to electrical drive systems, and more particularly to means for stopping electrical drive systems. The drive systems contemplated are those which are used in such applications as tandem cold reduction mills.

When the strip breaks or other emergencies arise 1t is extremely desirable to bring a tandem cold reduction mill to an immediate stop consistent with the mechanical limitations and energy absorption abilities of the motors and generators of the mill drive. The emergency stop of this nature should be made in as short a time as possible in order to minimize the resulting scrap material or to minimize the damage to the mill rolls. With the individual generator type of control now used for tandem cold reduction mills the reference bus voltage can be quickly reduced to zero and the individual stand regulators will force the stands to follow the reference bus closely and come to a stop. However, because of the diierences in stored energy for different mill speeds, it is not always possible to bring the mill to a stop with the maximum braking effort which can be exerted by the individual mill drives, if only one rate of deceleration can be obtained from the master reference rheostat. A second variable which must be considered is that the individual stands may be loaded or unloaded depending upon whether there is strip in the rolls or Whether the strip has left the rolls. If it were possible for any condition of operation to stop the mill at a rate consistent with the maximum braking current allowable for the mill stand having the highest stored energy, it would then be possible to stop the mill at all times at a maximum safe rate.

Accordingly, it is a main object of this invention to provide means for always stopping an electrical drive system at the maximum safe rate.

A more speciiic object of this invention is to stop an electrical drive system which comprises a plurality of stands at the maximum safe rate for the stand having the highest stored energy.

A still more specific object of this invention is to provide means for stopping an electrical drive system of the character referred to wherein the reference voltage for the individual stands is decreased at a rate which would tend to stop the system faster than the maximum permissible deceleration rate and an additional signal is applied to the reference voltage which is proportional to the regenerative current of the stand having the highest stored energy to limit the regenerative current of the stand motors.

The objects stated are merely illustrative. These and other objects will become more apparent from a study of the following specification and drawings in which:

Figure l is a diagrammatic showing of one embodiment of the emergency stop system applied to the electrical drive system; and

Fig. 2 shows diagrammatically another embodiment of the emergency stop system applied to the electrical drive system.

The embodiments of the emergency stop system were rice designed for use with the electrical drive system illustrated in detail in the application of Raymond W. Moore, William D. King, James T. Carleton and Loren F. Stringer, filed September 30, 1952, assigned to the Westinghouse Electric Corporation and given Serial No. 312,212. The drive system is shown diagrammatically in Fig. l and is explained brieliy herein, but for a more detailed explanation, reference should be had to the Moore et al. application previously referred to.

Generally, the drive system for one stand comprises a motor 10 having a iield winding 16 supplied from a suitable direct current source. The motor 10 is part of a Ward-Leonard drive which utilizes the main motor-supply generator 22 to supply the armature current for the motor 10. The motor control is obtained by comparing the voltage across generator 22 with the mill reference voltage, which appears across the leads 30 and 31. The difference between the voltage of the generator 22 and the mill reference voltage is used as a control quantity to regulate the voltage of the motor 10.

The mill reference voltage, which appears across the leads 30 and 31, is applied to control windings of the voltage regulator magnetic amplifier 32 and thus sets the basic excitation for the field windings 34 and 36 of the generator 22 to determine the speed of the mill stand motor 10. The voltage across the generator 22 is compared with the mill reference voltage by the circuit which comprises the lead 31 of the mill reference voltage supply, control windings on the voltage regulator magnetic amplilier 32, (IR) armature resistance drop compensating impedances 266 and 27o, lead 54, the armature of generator 22, to lead 30 of the mill reference voltage. Thus, any difference between the voltage of generator 22 and the mill reference voltage will cause a current to circulate around the comparator circuit just recited, and thereby cause a regulator current to flow in control windings on the voltage regulator magnetic amplifier 32 to regulate the voltage of generator 22.

The drive system for the iirst mill stand also has IR compensating magnetic ampliers 224. The control windings of the IR compensating magnetic amplifier 224 receives a signal from the lead 219 and the tap 223 adjustably coacting with potentiometer 218. The voltage supplied to the control winding of magnetic amplifier 224 is thus proportional to the motor load current. Therefore, the output of the IR compensating magnetic amplifiers 224 is determined by the IR drop in the armature circuit ofthe motor 10. The output of the IR compensating magnetic amplifiers is applied across the IR compensating resistors 266 and 270 which are in the comparator circuit previously recited and, therefore, the output of the IR compensating magnetic amplifiers 224 may be adjusted to have such au elect on the comparator circuit as to give the requisite compensation for the variations in the IR drop 1n the motor generator loop due to variations in current in the loop.

The mill stand No. 2 has components and circuits which lcorrespond to those `of mill stand No. 1 and they operate in the same manner, therefore, components for the mill stand No. 2 bear the same numerical designation with the addition of the reference character a as the corresponding components for the circuit for the mill stand No. l. For example, the motor of mill stand No. l is designated 10 and the motor of mill stand No. 2 is designated 10a and the generator of mill stand No. l is designated 22 and the generator of mill stand No. 2 is designated 22a.

With the individual generator type of control shown for the individual mill stand motors the mill can be stopped at a rate consistent with the maximum braking current allowable for the mill stand having the highest stored energy by forcing the mill reference voltage to grasses decrease at a rate which is always greater than the maximum permissible deceleration rate and applying an additional signal to increase the mill reference voltage which is proportional to the regenerative current of the mill stand having the highest stored energy. The means for accomplishing this is as follows. A reference exciter l is used to supply the mill reference voltage to the leads Siti and 3l. This exciter is driven at constant speed by any suitable means such as a constant speed motor (not shown). The reference exciter it has field windings 2 and 3 to furnish its excitation. A voltage is supplied between the leads 4 and 5 to the rheostat 6. The field Winding 2 of the reference exciter il receives its voltage from the rheostat o by means of the circuit which includes lead 7, field winding 2, resistor 8, emergency stop resistor and the lead lll. For normal operating conditions a switch l2 is closed which causes the emergency `stop contacter i3 to be energized and thus to shunt the resistor 9 which is in the circuit of the field winding 2 of the reference exciter ll. Thus, the reference exciter l is supplied with its normal excitation from the field Winding 2. When the emergency stop push button ES is depressed the circuit which contains the coil of the emergency stop contacter 13 is deenergized, the contacts of the emergency stop contacter open, and the resistor 9 is placed in series with the field winding 2 of the reference exciter l. The resistor 9 is of such a magnitude that the voltage of the reference exciter l is decreased at a very fast rate and the voltage regulator magnetic amplifiers 32 and 32a will cause the excitation of the generators 22 and 22a for their respective mill stands to follow the mill reference voltage closely. The value of the resistor 9 is of such a magnitude that the voltages of the individual generators 22 and 22a of the respective mill stands will reduce their voltage at a rate greater than the maximum permissible deceleration rate and, therefore, the individual mill stand motors l@ and lila will cause a regenerative current to flow in the individual motor generator loops.

In order to limit the decelerating current of the mill stand motor, a magnetic amplifier i4 is provided. The magnetic amplifier is connected in a conventional doubler circuit and has main windings 15 and l5', a biasing wind* ing 17 connected to a suitable source of D.C. voltage to set the point of operation of the magnetic amplifier and control winding l to determine the output of the magnetic amplifier. The magnetic ampliiier is supplied from the leads l@ and 2li and has its output connected across the field winding 3 of the reference exciter ll. The control winding i8 of the magnetic amplifier is supplied with a current which is determined by the regenerative current in the motor generator loop of the mill stand which has the highest stored energy. This is accomplished by providing circuits connected across the potentiometers Zlio and Zla. The potentiometers Zitti and Zla are connected across the commutating lields of the respective motors l@ and litio in the individual motor generator loops and, therefore, receive a voltage which is proportional to the current in the individual motor generator loops.

A differentiating circuit 2l, which contains resistors R1 and R2 and the capacitor C, is connected in series with the control winding 118 ofthe magnetic amplifier M'. The potentiometer 23S is connected across the series circuit which contains the control winding 18 and the diierentating circuit 2l by the leads 23 and 24. The potentiometer Zla is connected across the same series circuit by means of leads 25 and 2o. The leads 24 and 26 each have individual rectiiiers 27 and 28, respectively, connected in them and poled as shown.

rThus, if the stored energy in mill stand No. 2 is greater than the stored energy in mill stand No. l, its regenerative current will be greater upon deceleration than that in the motor generator loop of mill stand No. l. The voltage across the potentiometer 2ten then will be greater 'than the voltage across the potentiometer 218, thus, curim rent will liow from the potentiometer Ellen through the lead 26 and rectifier 2li through the diiterentiating circuit 2l and the control winding of the magnetic amplifier le through lead 25 to the potentiometer ZllSa. The current which tends to llow from potentiometer 2li; through the lead 24 and rectifier 27 is blocked by the current lion/ing due to the potential on the potentiometer lia. Therefore, the magnetic amplifier receives a signal which is proportional to the regenerative current of mill stand No. 2 or the mill stand having the highest stored energy. The control winding l of the magnetic amplifier le is wound such that a current through the circuit just recited in the sense shown tends to saturate the core and increase its output. An increase in the output of the magnetic amplifier 14 appears across the iield winding 3 of the reference exciter 1 and tends to increase the excitation of the reference exciter. The increase in excitation of the reference exciter is just sufficient to allow the mill reference voltage to decrease at that rate which will cause the mill to stop at a maximum safe rate.

if the mill stand No. l had a higher stored energy than mill stand No. 2, the system would operate in exactly the same manner as that just described except that the potentiometer 21S would have a greater voltage across it than the potentiometer 2Min and as a consequence would force current through the differentiating circuit 2l and the control Winding i3 of the magnetic amplifier The dififenentiating circuit 2l is used so that the current liowing through the control winding "iS of the magnetic ainplier ld is a measurerof the regenerative current and the rate of change of the regenerative current in the armature circuit of the motor of the mill stand having the highest stored energy. Actually it is the magnitude of regenerative current which is limited. The rate of change signal compensates for the time delays existing in the regulating system and eliminates instability.

The circuit of Fig. 2 shows diagrammatically another embodiment of the invention which will stop the mill at a rate consistent with the maximum current ailowaole for the mill stand having the highest stored energy. The mill stands shown have the same individual generator drive systems as shown and described for the mill stands of Fig. l and, therefore, corresponding components in Fig. 2 are given the same reference numerals as in Fig. l. The system of Fig. 2 also utilizes a reference exciter with two fields to supply the mill reference voltage and the means for reducing the excitation oi the reference exciter for an emergency stop is also the same and the corresponding components given the same reference characters as in Fig. l. The emergency stop system of `iig. 2 differs from that of Fig. l in that each individual mill stand has an individual current regulating magnetic ampliier to determine the excitation of the field winding 3 of the reference exciter l.. That is, the magnetic amplifier' il is associated with the mill stand No. l and the magnetic amplifier lila is associated with mill stand No. 2. The magnetic amplifier it is connected in the conventional doubler arrangement and has the main windings l and 42, a biasing Winding d3 and control winding 4e. The magnetic amplifier is supplied by an alternating current from the leads t5 and lio. The control winding 44 is connected in series with a differentiating circuit 1137 which contains resistors R3 and Ri and capacitor Ci. This series circuit is connected by leads d3 and across the potentiometer 218. Since the voltage across the potentiometer 218 is proportional to the current in thc armature circuit of the motor lill, a current will tlow through the differentiating circuit d? and the control winding 44 of the magnetic amplifier le which is a measure of the armature current of the motor l@ and its rate or" change and, therefore, the output of the magnetic amplifier itl will bedetermined accordingly. The magnetic amplifier lila is also of the conventional doubler type and has main windings lila and @261, a biasing winding 3a and a control winding 44a. The magnetic amplifier den is also supplied from an alternating current source connected between the leads 45 and 46. The control winding7 44a of the magnetic amplier 49a is connected in series with the differentiating circuit 47a which comprises resistors R and Re and the capacitor C2. This series circuit is connected by the leads 48a and 49a across the potentiometer 23fi. Conseouently, the magnetic amplifier 40a has its output determined in accordance with the armature current of the motor lila and its rate of change. Again the rate of change signal is used only to compensate for time delays, which exist in the regulating system and to thereby eliminate system instability.

Since the outputs of the magnetic amplifiers i0 and 40a are effectively connected in parallel at the input terminals Si) and 5i of the full wave bridge rectifier 52, the magnetic amplifier having the greatest output will in effect block the output of the other magnetic amplifier and, therefore, the voltage across the output terminals 53 and 54 of the full wave bridge rectifier 52 will be proportional to the current and rate of change of current in the armature of the motor for the mill stand which has the highest stored energy. The field winding 3 of the reference exciter l is connected directly across the output terminals 53 and 54 of the full wave bridge rectier 52 and, therefore, the reference exciter 1 receives additional excitation which limits the rate of decrease of its voltage and thereby insures that the mill will stop at the maximum rate which is consistent with the maximum braking current allowable for the stand having the highest stored energy.

lt will be readily seen that either of the systems shown and described here may be utilized where only one drive motor is used or where any number of drive motors are used. Two mill stands were shown only for purposes of illustration.

rl`he objects of invention have been accomplished by providing a means to stop a single motor at a rate consistent with the maximum braking current allowable or to stop a plurality of motors having a common mechanical load at the maximum rate consistent with the maximum current allowable for the stand having the highest stored energy.

While two best known and preferred embodiments of the invention have Leen shown and described in detail herein, it is to be particularly understood that equivalents are clearly within the scope of the invention.

We claim as our invention:

l. A control system for a motor connected in a loop circuit with a main generator which has its excitation determined by a reference voltage comprising, a reference exciter to determine the reference voltage, field excitation means fer said reference exciter, impedance means, means to insert said impedance means in series with said excitation for the reference and to remove said impedance means from the series circuit, said impedance means such a magnitude as to reduce the reference voltage at a rate which will tend to cause the motor to decelerate at a rate to cause regenerative current in excess of the permissible amount when inserted in series with said excitation means for said reference exciter, means for exciting the main generator with an additional signal which is proportional to the magnitude and rate of change of the regenerative current of the motor.

2. A contro system for a motor connected in a loop circuit with a main generator which has its excitation determined by a reference voltage comprising, means for reducing the magnitude of the reference voltage at a rate which t [tl tend to se the motor to decelerate at a rate to cause rnerative current in excess of the permissible amount, a reference exciter for supplying the reference voltage, means to feed baci; a signal to excitation means for said reference exciter which is proportional to the magnitude and rate of change of the regenerative current of the motor to limit the regenerative current.

3. A control system for a motor connected in a loop circuit with a main generator which has its excitation determined by a reference voltage comprising, a reference exciter to determine the reference voltage and the excitation of the main generator, means to reduce the excitation of the reference exciter at a rate which will cause the output voltage of the reference exciter to decrease at a rate which would tend to cause the motor to decelerate at a rate to cause regenerative current in excess of the permissible amount, means to feed back a signal to excitation means for said reference exciter which is proportional to the magnitude and rate of change of the regenerative current of the motor to limit the regenerative current.

4. A control system for a motor connected in a loop circuit with a main generator which has its excitation determined by a reference voltage comprising, a reference exciter to determine the reference voltage, field excitation means for said reference exciter, impedance means, means to insert said impedance means in series with said excitation means for the reference and to remove said impedance means from the series circuit, said impedance means having such a magnitude as to reduce the reference voltage at a rate which will tend to cause the motor to decelerate at a rate to cause regenerative current in excess of the permissible amount when inserted in series with said excitation means for said reference exciter, means to feed back a signal to excitation means for said reference exciter which is proportional to the magnitude and rate of change of the regenerative current of the motor to limit the regenerative current.

5. A control system for a motor connected in a loop circuit with a main generator which has its excitation determined by a reference voltage comprising, a reference exciter for supplying the reference Voltage, excitation means for the reference exciter controlling the excitation of said excitation means for said reference exciter in a sense to reduce the magnitude of the reference voltage at a rate which will tend to cause the motor to decelerate at a rate to cause regenerative current in excess of the permissible amount, differentiating circuit means connected to the armature of said motor to be supplied by a voltage proportional to the motor armature current, and circuit means connecting said differentiating circuit means to said excitation means for the reference exciter to control said reference exciter in accordance with the current in the differentiating circuit to limit the regenerative current.

6. A control system for a motor connected in a loop circuit with a main generator which has its excitation determined by a reference voltage comprising, controllable electrical means for producing a reference voltage for exciting the main generator, means controlling said controllable electrical means in a sense to reduce said reference voltage at a rate which would tend to cause the motor to decelerate at a rate to cause regenerative motor current in excess of the permissible amount, differentiating circuit means connected to the armature of the motor to be supplied by a voltage proportional to the motor armature current, and circuit means connecting said differentiating circuit means to said controllable electrical means to control said controllable electrical means in accordance with the current in the differentiating circuit to limit the regenerative current.

7. A control system for a motor connected in a loop circuit with a main generator which has its excitation* determined by a reference Voltage comprising, a reference exciter to determine the reference voltage, field excitation means for said reference exciter, impedance means,` means to insert said impedance means in series with said excitation means for the reference and to remove said impedance means from the series circuit, said irnpedance means having such a magnitude as to reduce the reference voltage at a rate which will tend to cause the motor to decelerate at a rate to cause regenerative; current in excess of the permissible amount when inserted in series with said excitation means for said reference:

exciter, a differentiating lcircuit connected to the armature of the motor tobe supplied by-.a voltage proportional to the ,motor `armature'current, and circuit means connecting saiddiierentiating circuit to said excitation means for the reference exciter' to control said reference exciter in accordance with the current in the differentiating circuit to limit the regenerative current.

8. A control system for a motor connected in a loop circuit with a main generator which has its excitation determined by a `reference voltage comprising, means for reducing the magnitude of the reference voi a rate which Will tend to cause the motor to decelerate at a rate to cause regenerative current in excess of the permissible amount, a reference exciter for supplying the reference voltage, a saturable core impedance device having main windings and a control Winding, a diiierew tiating circuit connected in series with the control v, l ing for said saturable core impedance device, said ditferentiating circuit andcontrol winding being connected to receive a voltage proportional to the current in tbe motor armature circuit, the output ofsaid saturable core impedance device being connected to determine the energization of said reference exciter in accordance with the signal on its control winding to limit the regenerative current.

9. A control system for a motor connected in a loop circuit with a main generator which its excitation determined by a reference voltage comprising, a reference exciter to determine the reference voltage and the excitation of the main generator, means to reduce the excitation of the reference exciter at a rate which will cause the output voltage of the reference exciter to decrease at a rate which would tend to cause the motor to decelerate at a rate lto cause regenerative current in excess of the permissible amount, a saturable core impedance device having main windings and a control Winding, a differentiating circuit connected in series with the control winding for said saturable core kimpedance device, said differentiating circuit and control Winding being connected to receive a voltage proportional to the current in the motor armature circuit, the output of said saturable core impedance device being connected to determine the energization of said reference exciter in accordance with the signal on its control winding to limit the regenerative current.

10. A control system for a motor connected in a loop circuit with a main generator which has its excitation determined by a reference voltage comprising, a reference exciter to determine the reference voltage, eld excitation means for said reference exciter, impedance means, means to insert said impedance means in series with said excitation means for the reference exciter and to remove said impedance means from the series circuit, said impedance means having such a magnitude as to reduce the reference voltage at a rate which will tend to cause the motor to decelerate at a rate to cause regenerative current in excess of the permissible amount when inserted in series with said excitation means for said reference exciter, a saturable core impedancedevice having main windings and a control winding, a differentiating circuit connected in series with the control Winding for said saturable core impedance device, said dierentiating circuit and control winding being connected to receive a voltage proportional to the current in the motor armature circuit, the output of said saturable core impedance device being connected to determine the energization of said reference exciter in accordance with the signal on its control Winding to limit the regenerative current.

ll. In a control system for a plurality of individual electric motors connected in individual loop circuits with individual Vmain generators which are excited from a common reference voltage comprising, a reference exciter to determine the reference voltage and the excitation of the individual main generators, means toreduce the excitation of the reference exciter at a rate which will cause v 8 the outputvoltage `ot Vthe reference to decrease yto 'such' an extent ,and Aatsuch varate that would tend tocausev the individual main motors to decelerate ata rate tocausc regenerative current in excess of the permissible amount, and means for exciting the individual main generators with an additional signal which is proportional to the magnitude and rate of change of the regenerative current oi the individual motor having the greatest rate of change of current and excess of current in its armature circuit. l2. In a controlsystem for a plurality of individual elec ic motors connected in individual loop circuits with individual main generators which are excited from a common reference voltage comprising, a reference exciter to determine the reference voltage, field excitation means tor said reference exciter, impedance means, means to insert s impedance means in series with said excitation means for tne reference and to remove said impedance means from the series circuit, said impedance means havingsuch a magnitude as to reduce the reference voltage at a rate which will tend to cause the motor to decelerate at a rate which will tend to cause regenerative current in excess of the permissible amount when inserted in series with said excitation means for said reference exciter, and means for exciting the individual main generators with an additional signal which is proportional to the magnitude and rate of change of the regenerative current of the individual motor having the greatest rate of change of current and excess of current in its armature circuit.

i3. in a control system for a plurality of individual electric motors connected in individual loep circuits with individual main generato-rs which are excited from a common. reference voltage comprising, a reference exciter to determine the reference voltage and the excitation of the individual main generators, excitation means for the reference exciter, means to reduce the excitation of the reference exciter at a rate which will cause the output voltage ofthe reference exciter to decrease to such an extent and at such a rate that would tend to cause the individual main motors to decelerate at a rate to cause regenerative current in excess of the permissible amount, and means for applying a signal to said excitation means for said reference exciter which is proportional to the magnitude and rate of change of the regenerative current of the individual moto-r having the greatest stored energy to imit and regulate the regenerative current in the armature circuit of each individual motor.

14. In a control system for a plurality of individual electric motors connected in individual loop circuits with individual main generators which are excited from a common reference voltage comprising, a reference exciter to determine the reference voltage, iield excitation means for said reference exciter, impedance means, means to insert said impedance means in series with said excitation means for the reference and to remove said impedance means from the series circuit, said impedance means having such a magnitude as to reduce the reference voltage at a rate which will tend to cause the motor to decelerate at a rate which Will tend to cause regenerative current in excess of the permissible amount when inserted in series with said excitation means for said reference exciter, and means for applying a signal to said excitation means for said reference exciter which is proportional to the magnitude and rate of change of the regenerative current of the individual motor having the greatest stored energy `to limit and regulate the regenerative current in the armature circuit of each individual motor.

l5. In a control system for a plurality of individual electric motors connected in individual loop circuits with individual main generators which are excited from a common reference Voltage comprising, a reference exciter to determine the reference voltage and the excitation of the individual main generators, excitation means for the reference exciter, means to reduce the excitation of the reference exciter at a rate which will cause the output voltage of lthe reference to decrease to an extent Aand v.at

a rate that would tend to cause the individual main motors to decelerate at a rate to cause regenerative current in excess of the permissible amount, individual differentiating circuits, each individual dilerentiating circuit being connected to be supplied by a voltage proportional to the armature current of its corresponding individual motor, and means for exciting said excitation means of said reference exciter in accordance with the current in the differentiating circuit which carries the most current to limit and regulate the regenerative current in the armatures of the individual motors.

16. In a control system for a plurality of individual electric motors connected in individual loop circuits with individual main generators which are excited from a common reference voltage comprising, a reference exciter to determine the reference voltage, lield excitation means for said reference exciter, impedance means, means to insert said impedance means in series with said excitation means for the reference and to remove said impedance means from the series circuit, said impedance means having such a magnitude as to reduce the reference voltage at a rate which will tend to cause the motor to decelerate at a rate which will tend to cause regenerative current in excess of the permissible amount when inserted in series with said excitation means for said reference exciter, individual diterentiating circuits, each individual diterentiating circuit being connected to be supplied by a voltage proportional to the armature current of its corresponding individual motor, and means for exciting said field excitation means of said reference exciter in accordance with the current in the differentiating circuit which carries the most current to limit and regulate the regenerative current in the armatures of the individual motors.

17. In a control system for a plurality of individual electric motors connected in individual loop circuits with individual main generators which are excited from a common reference voltage comprising, means for reducing the magnitude of the reference voltage at a rate which will tend to cause the individual motors to decelerate at a rate to cause a regenerative current in the armature circuit of each individual motor which is in excess of the permissible amount, a reference exciter for supplying the reference voltage, a saturable core impedance device having main windings and a control winding, a dilerentiating circuit connected in series with the control winding for said saturable core impedance device, individual sources of voltages for each individual motor to supply a voltage proportional to the current in the respective individual motor armature circuits, said individual voltages being connected in parallel with each other across the series circuit which contains said control winding of the saturable core impedance device andcach of said individual voltage sources having a blocking means in series with it which is poled to oppose a circulating current due to another voltage source, the outputfof said saturable core impedance device being connected to determine the energization of said reference exciter in accordance with the signal on its control winding to limit the regenerative current.

18. In a control system for a plurality of individual electric motors connected in individual loop circuits with individual main generators which are excited from a common reference voltage comprising, a reference exciter to determine the reference voltage and the excitation of the individual main generators, means to reduce the excitation of the reference exciter at a rate which will cause the output voltage of the reference to decrease to such an extent and at such a rate that-would tend to cause the individual main motors to decelerate at a rate to cause regenerative current in excess of the permissible amount, a saturable core impedance device having main windings and a control winding, a dilerentiating circuit connected in series with the control winding for said saturable core impedance device, individual sources of voltages for each individual motor to supply a voltage prof portional to the current in the respective individual motor armature circuit, said individual voltages being connected in parallel with each other across the series circuit which contains said control winding of the saturable core impedance device and each of said individual voltage sources having a blocking means in series with it which is poled to oppose a circulating current due to another voltage source, the output of said saturable core impedance device being connected to determine the energization of said reference exciter in accordance with the signal on its control winding to limit the regenerative current.

19. In a control system for a plurality of individual electric motors connected in individual loop circuits with individual main generators which are excited from a common reference voltage comprising, a reference exciter to determine the reference voltage, field excitiation means for said reference exciter, impedance means, means to insert said impedance means in series with said excitation means for the reference and to remove said impedance means from the series circuit, said impedance means having such a magnitude as to reduce the reference voltage at a rate which will tend to cause the motor to decelerate at a rate which will tend to cause regenerative current in excess of the permissible amount when inserted in series with said excitation means for said reference exciter, a saturable core impedance device having main windings and a control winding, a dilerentiating circuit connected in series with the control winding for said saturable core impedance device, individual sources of voltages for each individual motor to supply a voltage proportional to the current in the respective individual motor armature circuits, said individual voltages being connected in parallel with each other across the series circuit which contains said control winding of the saturable core impedance device and each of said individual voltage sources having a blocking means in series with it Which is poled to oppose a circulating current due to another voltage source, the output of said saturable core impedance device being connected to determine the energization of said reference exciter in accordance with the signal on its control winding to limit the regenerative current.

20. In a control system for a plurality of individual electric motors connected in individual loop circuits with individual main generators which are excited from a common reference voltage comprising, means for reducing the magnitude of the reference voltage at a rate which will tend to cause the individual motors to decelerate at a rate to cause a regenerative current in the armature circuit of eachindividual motor which is in excess of the permissible amount, a reference exciter for supplying the reference voltage, individual saturable core impedance devices each having main windings and a control Winding, individual differentiating circuits, each individual differentiating circuit connected in series with the control winding of a corresponding individual saturable core impedance device, the series combination being connected across a voltage source which is proportional to the armature current of its corresponding individual motor, the output of said individual saturable core impedance means being connected to excitation means of the reference exciter to determine its excitation to limit and regulate the regenerative current in the armature of the individual motorsin accordance with the armature current of the motor having the highest stored energy.

21. In a control system for a plurality of individual electric motors connected in individual loop circuits with individual main generators which are excited from a common reference voltage comprising, a reference exciter to determine the reference voltage and the excitation of the individual main generators, means to reduce the excitation of the reference exciter at a rate which will cause the output voltage of the reference to decrease to such an extent and at such a rate that would tend to cause the eritialt-'ingicircuits-,2 eachfindividal differentiating circuit f connectetinseries-with" thef control winding of a correspend-ing individuali saturable core impedance device, the series: combinationbeing connected across a voltage source;whichllisrproportional-to the armature current of it'fsi-corresponding'i-individual: motor, the output of said individuatsaturable'cor'ef impedance means being connected to excitation means ofthe reference exciter to determine-'its excitationuto limit and regulate the regenerative'vcurrentinz-the armatures of the individual motors in aceordancezwi'ththe armaturev current of the motor having thei'highest stored .energy.

22e Ima-,control system for a plurality of individual electric-motors connected in individual loop circuits with individual main generators which are excited from a common-referen`ce voltage comprising, a reference exciter to-v determine'the'reference.voltage, iield excitation means forsaid-reference-exciter, impedance means, means to insert-'saidI impedance means in series with said excitationimeans for the reference. and to remove said imp'edance' means from the' series circuit, said impedance means-having such a magnitude as to reduce the reference-voltage ata rate which will tend to cause the motor to' deceleratefat a rate which will tend to cause regenerative-current-in excess of vthe-A permissible amount when inserted-infseries with said excitation means for said reference'f'exciter, individual saturable core impedance devices eaelr-havinggmain: windings and a control winding, individual-differentiating" circuits, each individual differentiating. circuit;A connected inJseries with the control winding of ai corresponding individual saturable core impedaneer-device, the series combination being connected acrossfayfvoltager'sources:whiclr is proportiona-l to the armature; .currenu-ogits e corresponding individual motor,

tive motorsand having a single'output circuit, a variable` voltage control systemconnected to eachY motor armature to control` operation thereof, and circuit -means connecting saidsingle outputcircuit to-said variable voltage control systems tov-simultaneously control all of said variableV voltage control systems.

24.` Ax current limi-tn control v'for a-plurality ofi motors f comprising, respectivea variableivoltage-controle' systemsV- connectedftoisaid'- respectiveL motorsf to? control said", motors-andf-each having aninput circuit', referenc'zeK voltage 'electrical means having an output circuit connectedg to all said: input circuits of said variable voltage 'controiA systems and having input circuit means', a control circuit-j connected tov saidinput'circuit meansl of said reference',l voltage electrical'means-for reducing s-aidreference volt# ageat a rate to` causeregenerative motor currents, and'v a plurality of parallel connected similarly poledi rectifier circuits respectively connected to beenergized in dependenceofmotor current and having a single output. circuit connected to; said input circuit means of' said' reference voltage elctrical meansv to control said, refer-` ence voltage electrical means in a senser opposite'tosaid;` control circuit.

25. A current limit controlfor a plurality of motors comprising, a plurality of parallel connected magnetic' amplifiers having a single output circuit and having respective input circuits connected to be energized in dependence of armature currents ofthe respective motors, a variable voltage control system' connected to each motor armature to control energization thereof, each variable voltage control system having an input circuit, and circuit'means connecting said single output circuit of said magneticY ampliers to said respective input circuits of said variablevoltagecontrol systems tov simultaneously control all'of said variable voltage control systems.

26,v A current limit' control for a plurality of motorscomprising, a plurality of parallel connected magnetic' ampiitiers having a single output circuit and having respec-Y tive input circuits connected to be energized independence of armature currents of thefrespective motors, a variable voltage control systemzconnectedv to each motor amatureto control; energization thereof, eachvariablevoltageconf trol. system having an input' circuit;y a reference voltage" devicehaving an outputl circuitf connected toeach'input' circuitof said variablevoltage-control systems and having input circuit means, control means connected to saidfinput circuit meansv of saidJ referencevoltage d'evicefor"re-j ducingthe reference voltage at' a-ra'tesuiiicient to producei regenerative motor-'armature current, andicircuitfmean'sl connecting said single; output! circuit of saidfm'agnetic" amplifiers to said input circuit means of said referenceI voltage device to control. said reference voltageidev'icez in a sense opposite to said control means.

References Cited in theiilevof this patent UNTTED STATES PATENTS 2,154,279 Mu11er Apr.11,1939 

